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A common classification of drugs into Hard drugs and Soft drugs. Some drugs cannot be clearly classified that way, they are at the intersection of both circles.

Certain kinds of drugs that act on the nervous system are sometimes loosely classified. Usually there are the following categories:

Hard drugs are drugs that lead to physical addiction. Many countries do not allow people to make, sell or use some of them, other than for medical purposes. Examples of such drugs are opiates (heroin, hydrocodone (Vicodin), oxycodone (Oxycontin, Percocet), morphine), benzodiazepines (diazepam (Valium), alprazolam (Xanax), flunitrazepam (Rufilin), clonazepam (Klonopin), lorazepam (Ativan)), methamphetamine (meth), cocaine, alcohol, GHB, and nicotine. If it is legal to sell them, like with alcohol or nicotine, there are often taxes that need to be paid for them.

Soft drugs are not thought to cause physical addiction. Examples of soft drugs are cannabis, mescaline, psilocybin, LSD, ayahuasca, iboga, and DMT. While they do not cause physical addiction, some of them may still lead to psychological dependence.

Some drugs cannot be classified that way, as they have characteristics of both hard and soft drugs. Examples for such drugs are MDMA (known as Ecstasy/Molly), ketamine, MXE (methoxetamine), PCP (phencyclidine), DXM (dextromethorphan), synthetic cannabis (known as Spice), amphetamine and caffeine.

In the Netherlands, there are situations where the use of some "soft drugs" is tolerated.